<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #d1{
            width: 100px;
            height: 100px;
            background-color: red;
        }
    </style>
</head>
<body>
    <div id="d1">

    </div>
    <script>
        let d1 = document.getElementById("d1");
        d1.onmouseover = function(){
            d1.style.width = "200px";
            d1.style.height = "200px";
        }
        d1.onmouseout = function(){
            d1.style.width = "";
            d1.style.height = "";
        }
    </script>
</body>
</html>